Mechanical actuators are typically used to provide controlled, precise linear movement. One form of mechanical actuator is the electro-thermal actuator which can provide precise movement over one or two inches. This type of actuator has a working or internal medium in a sealed variable volume chamber in which one movable wall of the chamber, which is to execute the precise movement, can be acted upon by the medium. By means of changes created in the medium temperature, pressure increases and decreases, respectively, are applied to the movable wall as a result of controlled expansion and contraction of the medium. The changes in chamber volume are converted to a linear or mechanical output to perform work on an external device.
In an electro-thermal actuator, the working medium may be a fluid. Changes in the fluid pressure in the chamber are effected by applying energy to the working fluid, which energy may be in the form of thermal energy, such as heat. The application of heat causes the fluid to undergo a phase change from liquid to gas. Application of heat is typically effected using a resistance-type heater placed within the fluid chamber. Application of electrical current to the resistive element of the heater causes the element to heat up, thereby imparting heat to the working fluid in proximity to the heater.
Electro-thermal actuators which utilize thermal energy to expand an expansible working medium are known. One such type of actuator is disclosed in U.S. Pat. No. 4,029,041 to Huebscher et al. This reference discloses an expanding liquid thermal actuator. In this actuator, a heater is energized in order to heat and thereby expand the volume of a working fluid. The increased pressure of the working fluid acts to move a piston. Accurate control of the heating mechanism and the resulting piston movement, however, is not disclosed. U.S. Pat. No. 4,887,429 to Birli, Sr. et al. describes an electro-thermal actuator having a flexible separator partitioned between a fluid chamber and a heater chamber. The separator is flexible in order to allow it to withstand vibrations. Here also, there is no precise piston control.
U.S. Pat. No. 4,759,189 to Stropkay et al. discloses an electro-thermal actuator which utilizes a positive temperature coefficient (PTC) heater in series with a conventional resistance heater. The conventional heater provides a fast initial response, while the PTC heater subsequently acts as a current limiting device to control the amount of current being applied to the heater. The PTC heater is also used to somewhat regulate the temperature of the working fluid, since the temperature of the PTC is limited to its "anomaly temperature" or the temperature at which the transition from low resistance to extremely high resistance occurs. U.S. Pat. No. 4,104,507 to Tisone et al. describes an electro-thermal actuator with a PTC heater. However, in neither of these patents is there a provision for selectively controlling temperature to thereby control the extent of movement of the actuator.